This invention relates generally to measurement of capacitance, and in particular to improved measurement of capacitance using a constant current source in a charge measurement system.
Prior art methods of measuring capacitance have included capacitance bridges and other precision instrumentation that is usually complex and expensive. In U.S. Pat. No. 5,073,757 to Richard E. George and assigned to Fluke Corporation, a method of measuring small capacitances was disclosed in which an unknown capacitor was discharged and then fully charged through a reference resistor, while at the same time a current proportional to the charging current was accumulated on the storage capacitor of a dual-slope analog-to-digital converter (ADC). The charge was then removed over a period of time dictated by the amount of stored charge, and the time was measured to give an indication of capacitance value. Because the capacitor had to fully charge within the integrating cycle of the dual slope ADC, that is, the capacitor had to charge for at least five RC time constants, only small capacitance values, e.g., on the order of five microfarads or less, could be measured.
This limitation was immediately recognized, resulting in the system disclosed in U.S. Pat. No. 5,136,251 to Richard E. George et al. and assigned to Fluke Corporation. Rather than attempting to charge the unknown capacitor in one cycle of a dual-slope integrating ADC, the capacitor was at least partially discharged and then charged over a multiplicity of ADC cycles. On each successive ADC cycle, referred to as "minor cycles" since the measurement was incomplete until the final ADC cycle, the amount of charge stored on the ADC's storage capacitor for that cycle was measured while the accumulated charge on the unknown capacitor built up to the fully charged condition. On each minor cycle of the ADC, when the measurement for that cycle was made the charging current to the unknown capacitor was suspended. The ADC stored progressively less charge on the storage capacitor for each successive minor cycle because the RC charge curve of the unknown capacitor began to flatten out, but the end result was to accumulate the full charge over a number of ADC cycles.
The drawbacks to these prior art capacitance measurement techniques include inordinately long measurement times because the unknown capacitor has to charge fully. This is particularly annoying when determining the value of large capacitors because the measuring instrument appears "dead" to the user during slow measurements. Also, relying on RC time constants results in inaccuracies because the RC charge curve becomes asymptotic in approaching the final voltage. This is particularly true for measuring large capacitors because the RC charge curve is broken into progressively smaller pieces.
Another factor making capacitance measurements difficult, particularly in hand-held digital multimeters, is that dual slope ADCs are being replaced by other, faster ADCs, such as multislope ADCs. Dual slope ADCs integrate an unknown quantity, such as voltage, over a fixed period of time, and then in what is known as a "de-integrate cycle" measure the length of time it takes to remove the integrated and stored quantity. Multislope ADCs exhibit faster measurements because the dynamic range of the integrated and stored quantity (voltage) being measured is reduced, with known charge being added or removed during the integrate cycle to keep the accumulated quantity within a narrow input window. This of course results in a substantially reduced "de-integrate cycle" in which the final quantity is measured and algebraically summed with the known added or subtracted charge. Often the voltage charge curve required to fully charge an unknown capacitor in order to make an accurate measurement either is not compatible with the timing and mechanics of the ADC integrate cycle, or the voltage is outside the dynamic window of the ADC. An example of a multislope ADC is disclosed in U.S. Pat. No. 5,321,403 to Benjamin Eng, Jr., et al. and assigned to Fluke Corporation.